VEGETATIVE ORGANS OF PLANTS. 



295 



partitions, and become continuous channels or ducts. 

 (Sieve-cells, p. 303. ) The cambium is the seat of growth 

 by cell-formation. Accordingly, when a vascular bun- 

 dle has attained maturity, it no longer possesses a cam- 

 bium. 



To complete our view of the vascular bundle, Fig. 50 

 represents a vertical section made at right angles to the 

 last, cutting two large ducts, b, b; a, a is cell- tissue; 

 c 



Fig. 50. 



c, c are bast or wood-cells less thickened by interior 

 deposition than those of Fig. 49 ; d is a ring and spiral 

 duct ; b, b are large dotted ducts, which exhibit at g, g 

 the places where they were once crossed by the double 

 membrane composing the ends of two adhering cells, by 

 whose absorption and removal an uninterrupted tube 

 has been formed. In these large dotted ducts there 

 appears to be no direct communication with the sur- 

 rounding cells through their sides. The dots or pits 

 are simply very thin points in the cell-wall, through 

 which sap may soak or diffuse laterally, but not flow. 



